Fluid flow control valves used in the manufacture of devices, such as semiconductors, must be capable of adjusting precise fluid flow rates through an orifice as small as 40 microns. The structure of a valve chamber of such a fluid control valve consists of a valve disk interposed between a valve head and a valve seat. Vertical displacement of the valve head in relation to the valve seat creates an orifice through the valve disk to enable fluid from a primary (input) side to exit the valve at a secondary (output) side at a precise flow rate.
The flow rate (Q) of the control valve is proportional to the size of the orifice (1) and the diameter (D) of the valve seat and is expressed by the following equation: ##EQU1## where; K is a constant
g is gravitational acceleration PA1 Y is the specific gravity of a fluid PA1 P1 is a primary side pressure and, PA1 P2 is a secondary side pressure.
It can be seen that flow rate can be varied by changing at least one of the following: the size of the orifice (1), the size of the valve seat diameter (D), or the pressure difference (P1-P2).
In a conventional flow valve, the size of the orifice (1) must remain as small as 40 microns and the diameter of the valve seat (D) is restricted by the size of the flow valve. The pressure (P1-P2) cannot be increased, because as the pressure difference between the primary and secondary sides is increased, the valve head tends to be drawn by suction towards the valve seat, reducing the flow rate unpredictably, precluding accurate control of the flow rate. The increased suction can also apply excessive force to an actuator shaft that holds the valve head against the valve seat.
When the valve seat diameter (D) is enlarged, the pressure applied to the valve disk from the primary side increases, causing stress on the actuator that can result in premature breakdown.
For the reasons stated, a major disadvantage of conventional flow valves is that they cannot be adjusted to vary flow rate and must be replaced or modified for various job requirements.
U.S. Pat. No. 4,666,126 teaches a control valve wherein a flow passage is formed in a valve disk while two projecting ridges are provided on a peripheral edge of the lower surface of the valve disk. Fluid exits the orifice between these projecting ridges into the flow passage to widen the range of flow rate control. In this control valve, however, the widened range of flow rate control is attained by enlarging the area of a plane inside the inner projecting ridge. This results in constriction in the control valve flow passage that necessitates modification of the structure or replacement of the control valve to accommodate a higher flow rate.